[dagr] Re: random question

And a bit of progress, I put the FFT sink in Float mode after the
demodulator and sure enough there is the audio 0 - about 15kHz and then a
nice big spike at 19khz (my pilot wave). I am of course missing the upper
end of the demodulated signal so next up we recover that.

The last time I had to do anything similar, which was a very long time ago,
I used the Goertzel algorithm to do a single-bin DFT, which basically gives
you a single bin of an FFT. For a small number of bins, doing multiple
single-bin DFTs (with Goertzel or otherwise) is more efficient than the FFT.

It appears that GNU Radio does have a Goertzel block listed under Fourier
Analysis.

The Goertzel algorithm is often described as being useful for DTMF
decoding, but to the best of my knowledge most real-word DTMF decoders do
NOT use it. The problem is that Goertzel is good enough for an
unsophisticated DTMF decoder, but usually not sufficient for one that
actually meets all the Bellcore and ITU-T specifications.